We will present recent demonstrations of QKD at short optical wavelengths (532nm, 810 nm), and how it offers unique applications such as operation with ultra-high channel losses of 60dB, or entanglement based QKD within existing - and fully active - IT networks.
Short-wavelength QKD has been around ever since experiments have been performed, and is therefore widely studied. One important benefit for systems operating at that wavelength is the use Si-avalanche-photo-diodes for the photon detection. These offer high detection efficiency, very low dark counts and free-running operation.
The visible wavelength range is ideally suited for free-space applications, because the transmission through atmosphere is very good and the beam diffraction is still small. The use of such systems is to exchange quantum keys or entangled photons between mobile sites, and in the future even satellites. We implemented a system using the best available Si-detectors, an advanced and high-precision timing analysis system, and an ultrafast and modulated faint-laser source to achieve operation at ultra-high losses of 60dB. Our system shows a viable approach for operating under difficult situations such as the uplink of photons to a satellite, as well high-background, and these possible applications will be briefly reviewed.
In terms of fiber optic based transmissions, the current systems usually favor telecom wavelength over the short wavelengths, because existing IT infrastructure is single mode for telecom signals. This comes at the cost of needing sophisticated, and noisier photon detectors based on InGaAs. However, it has been established that also short wavelengths can be utilized over telecom optical fibers. The use of spatial and temporal filters must be implemented to suppress higher order modes in the optical fibers. We will show our recent experimental demonstration of entanglement based QKD, where the photon pairs created from a simple continuous wave operated entangled photon source can be distributed symmetrically over telecom optical fiber, and generated high quality secure keys. In a further experiment we show that this system directly allows the parallel transmission of QKD and classical telecom signals over the same optical fibers.